76 research outputs found
Diffusion-induced dissipation and mode coupling in nanomechanical resonators
We study a system consisting of a particle adsorbed on a carbon nanotube
resonator. The particle is allowed to diffuse along the resonator, in order to
enable study of e.g. room temperature mass sensing devices. The system is
initialized in a state where only the fundamental vibration mode is excited,
and the ring-down of the system is studied by numerically and analytically
solving the stochastic equations of motion. We find two mechanisms of
dissipation, induced by the diffusing adsorbate. First, short-time correlations
between particle and resonator motions means that the net effect of the former
on the latter does not average out, but instead causes dissipation of
vibrational energy. For vibrational amplitudes that are much larger than the
thermal energy this dissipation is linear; for small amplitudes the decay takes
the same form as that of a nonlinearly damped oscillator. Second, the particle
diffusion mediates a coupling between vibration modes, enabling energy transfer
from the fundamental mode to excited modes, which rapidly reach thermal
equilibrium.Comment: 8 pages, 7 figure
Noise-tunable nonlinearity in a dispersively coupled diffusion-resonator system using superconducting circuits
The harmonic oscillator is one of the most widely used model systems in
physics: an indispensable theoretical tool in a variety of fields. It is well
known that otherwise linear oscillators can attain novel and nonlinear features
through interaction with another dynamical system. We investigate such an
interacting system: a superconducting LC-circuit dispersively coupled to a
superconducting quantum interference device (SQUID). We find that the SQUID
phase behaves as a classical two-level system, whose two states correspond to
one linear and one nonlinear regime for the LC-resonator. As a result, the
circuit's response to forcing can become multistable. The strength of the
nonlinearity is tuned by the level of noise in the system, and increases with
decreasing noise. This tunable nonlinearity could potentially find application
in the field of sensitive detection, whereas increased understanding of the
classical harmonic oscillator is relevant for studies of the
quantum-to-classical crossover of Jaynes-Cummings systems.Comment: 8 pages, 8 figure
Diffraction and near-zero transmission of flexural phonons at graphene grain boundaries
Graphene grain boundaries are known to affect phonon transport and thermal
conductivity, suggesting that they may be used to engineer the phononic
properties of graphene. Here, the effect of two buckled grain boundaries on
long-wavelength flexural acoustic phonons has been investigated as a function
of angle of incidence using molecular dynamics. The flexural acoustic mode has
been chosen due to its importance to thermal transport. It is found that the
transmission through the boundaries is strongly suppressed for incidence angles
close to 35. Also, the grain boundaries are found to act as diffraction
gratings for the phonons
Scattering of flexural acoustic phonons at grain boundaries in graphene
We investigate the scattering of long-wavelength flexural phonons against
grain boundaries in graphene using molecular dynamics simulations. Three
symmetric tilt grain boundaires are considered: one with a misorientation angle
of displaying an out-of-plane buckling 1.5 nm high and 5 nm wide,
one with a misorientation angle of and an out-of-plane buckling 0.6
nm high and 1.7 nm wide, and one with a misorientation angle of
and no out-of-plane buckling. At the flat grain boundary, the phonon
transmission exceeds 95 % for wavelengths above 1 nm. The buckled boundaries
have a substantially lower transmission in this wavelength range, with a
minimum transmission of 20 % for the boundary and 40 % for the
boundary. At the buckled boundaries, coupling between flexural and
longitudinal phonon modes is also observed. The results indicate that
scattering of long-wavelength flexural phonons at grain boundaries in graphene
is mainly due to out-of-plane buckling. A continuum mechanical model of the
scattering process has been developed, providing a deeper understanding of the
scattering process as well as a way to calculate the effect of a grain boundary
on long-wavelength flexural phonons based on the buckling size.Comment: 11 pages, 14 figure
Nonlinear phononics using atomically thin membranes
Phononic crystals and acoustic meta-materials are used to tailor phonon and
sound propagation properties by facilitating artificial, periodic structures.
Analogous to photonic crystals, phononic band gaps can be created, which
influence wave propagation and, more generally, allow engineering of the
acoustic properties of a system. Beyond that, nonlinear phenomena in periodic
structures have been extensively studied in photonic crystals and atomic
Bose-Einstein Condensates in optical lattices. However, creating nonlinear
phononic crystals or nonlinear acoustic meta-materials remains challenging and
only few examples have been demonstrated. Here we show that atomically thin and
periodically pinned membranes support coupled localized modes with nonlinear
dynamics. The proposed system provides a platform for investigating nonlinear
phononics
Multi-phonon relaxation and generation of quantum states in a nonlinear mechanical oscillator
The dissipative quantum dynamics of an anharmonic oscillator is investigated
theoretically in the context of carbon-based nano-mechanical systems. In the
short-time limit, it is known that macroscopic superposition states appear for
such oscillators. In the long-time limit, single and multi-phonon dissipation
lead to decoherence of the non-classical states. However, at zero temperature,
as a result of two-phonon losses the quantum oscillator eventually evolves into
a non-classical steady state. The relaxation of this state due to thermal
excitations and one-phonon losses is numerically and analytically studied. The
possibility of verifying the occurrence of the non-classical state is
investigated and signatures of the quantum features arising in a ring-down
setup are presented. The feasibility of the verification scheme is discussed in
the context of quantum nano-mechanical systems.Comment: 23 pages, 8 figures; Minor revisions; Accepted for publication in NJ
Nanoscale Elasto-Capillarity in the Graphene-Water System under Tension: Revisiting the Assumption of a Constant Wetting Angle
Wetting highly compliant surfaces can cause them to deform. Atomically thin materials, such as graphene, can have exceptionally small bending rigidities, leading to elasto-capillary lengths of a few nanometers. Using large-scale molecular dynamics (MD), we have studied the wetting and deformation of graphene due to nanometer-sized water droplets, focusing on the wetting angle near the vesicle transition. Recent continuum theories for wetting of flexible membranes reproduce our MD results qualitatively well. However, we find that when the curvature is large at the triple-phase contact line, the wetting angle increases with decreasing tension. This is in contrast to existing macroscopic theories but can be amended by allowing for a variable wetting angle
Strong mechanically-induced effects in DC current-biased suspended Josephson junctions
Superconductivity is a result of quantum coherence at macroscopic scales. Two
superconductors separated by a metallic or insulating weak link exhibit the AC
Josephson effect - the conversion of a DC voltage bias into an AC supercurrent.
This current may be used to activate mechanical oscillations in a suspended
weak link. As the DC voltage bias condition is remarkably difficult to achieve
in experiments, here we analyse theoretically how the Josephson effect can be
exploited to activate and detect mechanical oscillations in the experimentally
relevant condition with purely DC current bias. We unveil for the first time
how changing the strength of the electromechanical coupling results in two
qualitatively different regimes showing dramatic effects of the oscillations on
the DC current-voltage characteristic of the device. These include the
apperance of Shapiro-like plateaux for weak coupling and a sudden
mechanically-induced retrapping for strong coupling. Our predictions,
measurable in state of the art experimental setups, allow the determination of
the frequency and quality factor of the resonator using DC only techniques.Comment: 10 pages, 6 figure
FPU physics with nanomechanical graphene resonators: intrinsic relaxation and thermalization from flexural mode coupling
Thermalization in nonlinear systems is a central concept in statistical
mechanics and has been extensively studied theoretically since the seminal work
of Fermi, Pasta and Ulam (FPU). Using molecular dynamics and continuum modeling
of a ring-down setup, we show that thermalization due to nonlinear mode
coupling intrinsically limits the quality factor of nanomechanical graphene
drums and turns them into potential test beds for FPU physics. We find the
thermalization rate to be independent of radius and scaling as
, where and
are effective resonator temperature and prestrain
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